FIG. 27 shows an exemplary configuration of a conventional optical-wireless hybrid transmission system. FIG. 28 shows an exemplary configuration of an optical receiver. FIG. 29 shows exemplary frequency spectra of respective signals in the conventional optical-wireless hybrid transmission system.
A central office 100 is equipped with an optical transmitter 10 and plural optical receivers 20. In the optical transmitter 10, an optical splitter 12 splits, into plural signals, a single-mode optical signal 0a (center frequency: fC) that is output from a single-mode optical source 11. The split optical signals 0a are transmitted as optical carrier signals to plural base stations 300 via optical transmission lines 201 and input to optical modulators 301, respectively. Only one base station 300 is shown here.
On the other hand, in a wireless terminal 400, an electrical carrier signal 0c (frequency fRF1) input from an oscillator 401 to a modulator 402 is intensity-modulated according to transmit-data 0b and a resulting RF signal 0d is transmitted from an antenna 403 to the base station 300. In the base station 300, the RF signal 0d modulated according to the transmit-data 0b is received by an antenna 302 and input to the optical modulator 301. The optical modulator 301 optical-intensity-modulates the optical carrier signal 0a supplied from the optical transmitter 10 according to the received RF signal, and a modulated optical signal 0e is amplified optically by an optical amplifier 303 and transmitted to the optical receiver 20 of the central office 100 via an optical transmission line 202. The plural optical receivers 20 are connected to the corresponding base stations 300, respectively.
In the optical receiver 20, the modulated optical signal 0e transmitted from the base station 300 is amplified optically by an optical amplifier 21 and then DSB-SC-modulated (DSB-SC: double-sideband suppressed-carrier) by using an electrical carrier signal 0f (frequency: fRF2/2) that is input from an oscillator 22 to the optical modulator 23. Only an optical signal 0h including prescribed two waves is extracted from an output optical signal 0g of the optical modulator 23 by an optical filter 24, and then square-law-detected by a photodetector 25.
The frequency fRF2/2 of the electrical carrier signal 0f that is output from the oscillator 22 is set at a half of a frequency that is lower than the frequency fRF1 of the RF signal 0d by an arbitrary intermediate frequency fIF, whereby an electrical signal 0i having the arbitrary intermediate frequency fIF is obtained as an output of the photodetector 25 and demodulated by an electrical demodulator 26. In this manner, receive-data 0j corresponding to the transmit-data 0b that is transmitted from the wireless terminal 400 is obtained without the need for using a receiver of an RF signal frequency band (refer to Non-patent document 1).
Non-patent document 1: Toshiaki Kuri and Ken-ichi Kitayama, “New Photonic Downconversion Technique with Optical Frequency Shifter for 60-GHz-Band Fiber-Radio Uplink Systems,” Proceedings of the 2002 General Assembly of the Institute of Electronics, Information and Communication Engineers (IEICE), IEICE, Mar. 7, 2002, C-14-13.